CVD synthesis of carbon nanotubes

ABSTRACT

A method of production of carbon nanoparticles comprises the steps of: providing on substrate particles a transition metal compound which is decomposable to yield the transition metal under conditions permitting carbon nanoparticle formation, contacting a gaseous carbon source with the substrate particles, before, during or after said contacting step, decomposing the transition metal compound to yield the transition metal on the substrate particles, forming carbon nanoparticles by decomposition of the carbon source catalysed by the transition metal, and collecting the carbon nanoparticles formed.

CROSS REFERENCE TO RELATED APPLICATION

This is the U.S. National Stage of PCT/GB2003/003086, filed Jul. 16,2003, which in turn claims priority to Great Britain Application No.0216654.4, filed Jul. 17, 2002, both of which are incorporated herein intheir entirety by reference.

The present invention relates to methods of synthesis of carbonnanoparticles, and to the carbon nanoparticles thus produced.

Carbon nanoparticles have received a great deal of attention since thediscovery of the C₆₀ buckminsterfullerene molecule (H. W. Kroto, J. R.Heath, S. C. O'Brien, R. F. Curl and R. E. Smally, Nature 318, 162(1985)) and the carbon nanotube (S. Ijima, Nature 354, 56 (1991)).Carbon nanoparticles are typically 1 to 100 nm in at least onedimension, carbon nanotubes however being up to a few millimeters inlength. The explosion in C₆₀ research in the early 1990s was driven bythe production of large quantities (few milligrams) of the material byKrastchmer et al. (W. Kratschmer, L. D. Lamb, K. Fostiropoulos and D. R.Huffman, Nature 347, 354 (1990)) using a high pressure arc dischargemethod.

The remarkable mechanical and electronic properties exhibited by carbonnanotubes have encouraged efforts to develop mass production techniques.As a result, carbon nanotubes are becoming increasingly available, andmore attention from both academia and industry is focused on theapplication of carbon nanotubes in bulk quantities. These opportunitesinclude the use of carbon nanotubes as a conductive filler in insulatingpolymer matrices, and as reinforcement in structural materials. Otherpotential applications exploit the size of carbon nanotubes as atemplate to grow nano-sized, and hence ultra-high surface-to-volumeratio, catalysts or aim to combine carbon nanotubes to formnano-electronic elements.

The high cost and low production volume of carbon nanotubes are atpresent prohibitive for them to be used as a filler material in mostlarge-scale structural and electrical applications. Presently, severalindustrial and governmental projects are underway to mass produceseveral kilograms of single and multi-walled carbon nanotubes in acost-effective manner. For example, the National Institute of Materialsand Chemical Research (NIMCR) and Showa Denko K. K. in Japan recentlyannounced a project to develop a mass-production method to produceseveral hundred kilograms of nanotubes per day.

Carbon nanotubes have been produced previously using various approachesincluding the laser or arc-discharge ablation of a carbon/catalystmixture target.

For larger scale synthesis, the most promising methods have been basedon chemical vapour deposition (CVD). CVD typically uses a cheapfeedstock and has relatively low energy requirements, and has thereforeattracted interest for the purposes of bulk synthesis. In CVD methods, acarbon containing gas is decomposed at high temperatures under theinfluence of a finely divided catalyst (usually iron, nickel, cobalt orother transition metals or-alloys).

Catalyst particles may be manufactured in situ by the decomposition ofmetalloorganic compounds or may be inserted into the CVD furnace on afixed substrate (W. E. Alvarez et al., Carbon 39 (2001) 547-558;WO00/17102; WO00/73205). For the growth of small nanotubes andsingle-walled nanotubes in particular, very small metal clusters (around1 nm) are required.

The catalyst may be in the form of a fragmented surface layer on aporous or non-porous macroscopic substrate (Ren et al., Bower et al., VI Merkulov, D H Lowndes, Y Y Wei et al., Andrews et al., and Cui etal.). As described in Kanzow et al, the catalyst may be a laser ablatednickel target exposed to a flow of reactant gas.

Alternatively, the catalyst may be in finely divided form. InWO00/17102, the catalyst is constituted by nanometer sized metalparticles supported on larger (10-20 nm) alumina particles. Theparticles are placed in the centre of a furnace and the carboncontaining gas is passed over them.

In WO00/73205, catalyst particles comprising two different metalssupported on silica, alumina, magnesia, zirconia or zeolite are used,again placed in a tube within a furnace. It is also suggested that themetallic catalytic particles may be continuously fed to the furnace.

Many catalyst precursors known in the art are initially contaminatedwith carbon-containing surfactants (e.g. Pluronic 123™) derived fromprecursor solutions. Carbon is removed by calcination at elevatedtemperatures (typically around 500° C.) in air, to produce an oxide.Alternatively, oxides or other inorganic salts which decompose to oxides(e.g. nitrates and carbonates) are used directly as catalyst precursors.In either case, it is then usual to reduce the oxide to the metal in afurther step (typically 700° C. in hydrogen).

In WO00/26138, catalyst nanoparticles are continuously produced within afurnace in the presence of reactant gas by decomposing a gaseouscatalyst precursor (normally Fe(CO)₅) in the presence of a ‘nucleationagency’. This may be a laser which provides some or all of the energyneeded for photolysis of the catalyst precursor, or it may be aprecursor moiety that stimulates clustering of catalyst atoms bydecomposing more rapidly or binding to itself more strongly afterdissociation. Typically, this is Ni(CO)₄.

A continuing problem in this art is the control over the extent of theproduction of multi-walled nanotubes in preference to single wallednanotubes and the control of the diameter of the tubes. In CVDsynthesis, fine structures, such as single walled nanotubes require veryfine catalyst particles with diameters similar to that of thesynthesised material (typically about 1 nm). Maintaining the requiredcatalyst particle size generally requires the use of a substrate to actas a carrier material to stabilise the catalyst itself. However, theproduction of very fine supported catalyst particles prior to use in thenanotube synthesis is generally complex and expensive involving forexample aggressive reagents and supercritical drying. Substantialproblems arise in preventing nanoparticles from coalescing prematurelyand the synthesis of such particles is not suitable for scaled-upproduction.

The production of catalyst particles in situ in the reaction zone as inWO00/26138, where the catalyst particles are essentially unsupported, orwhere the nucleation of catalyst clusters is enhanced by the presence ofNi species suffers from a lack of particle size control. Since theparticles are growing, the time at which they initiate nanotube growthmay be critical.

Because the nucleation sites are formed in situ from individual Ni atomsand comprise only a few atoms (2 to 5 atoms) in total, the processoffers little control over the size of the nucleating “particle”, or ofthe size of the final catalyst clusters. There is no controlledtemplating of the catalyst by the structure of the substrate.

In a first aspect, the present invention provides a method of productionof carbon nanoparticles, comprising the steps of:

-   -   providing on substrate particles a transition metal compound        which is decomposable to yield the transition metal under        conditions permitting carbon nanoparticle formation;    -   contacting a gaseous carbon source with the substrate particles;    -   before, during or after the contacting step, decomposing the        transition metal compound to yield the transition metal on the        substrate particles and forming carbon nanoparticles by        decomposition of the carbon source catalysed by the transition        metal;    -   and    -   collecting the carbon nanoparticles formed.

The transition metal compound may contain more than one metallicelement. Alternatively or additionally, a mixture of transition metalcompounds may be used, and in this case the different transition metalcompounds may contain different metallic elements.

Preferably, the transition metal compound is a transition metal salt.More preferably, the transition metal salt is a transition metal formateor oxalate.

The transition metal compound may be a transition metal carbonyl, morepreferably a multi metal atom carbonyl. The transition metal carbonylsis preferably non-volatile, in the sense that it does not evaporate orsublime before decomposition takes place. The transition metal carbonylmay be a neutral compound or a salt. Preferred multi metal atomcarbonyls contain from 13 to 55 metal atoms, for example a Ni₃₈ carbonylcluster. Suitable transition metal carbonyls for use in the inventionmay be made by the methods described in “Preparation, Characterisationand Performance of Encapsulated Copper-Ruthenium Bimetallic CatalystsDerived from Molecular Cluster Carbonyl Precursors.” D. S. Shephard, T.Maschmeyer, G. Sankar, J. M. Thomas, D. Ozkaya, B. F. G. Johnson, R.Raya, R. D. Oldroyd, R. G. Bell, Chemistry Eu. J., 1998, 4, 1227;“Site-Directed Surface Derivatisation of MCM-41: Use of HRTEM andMolecular Recognition for Determining the Position of Functionalitywithin Mesoporous Materials.” D. S. Shephard, W. Zhou, T. Maschmeyer, J.M. Matters, C. L. Roper, S. Parsons, B. F. G. Johnson, M. J. Juer,Angew. Chem. Int. Ed., 1998, 37, 2718; “Preparation and Characterisationof a Highly Active Bimetallic (Pd—Ru) Nanoparticle HeterogeneousCatalyst.” D. S. Shephard, R. Raja, G. Sankar, S. Hermans, S. Bromley,J. M. Thomas, B. F. G. Johnson, Chem. Comm. 1999, 1571; and“Supramolecular ordering of Ruthenium Cluster Carbonyls in MesoporousSilica.” W. Zhou, D. S. Shephard, J. M. Thomas, T. Maschmeyer, B. F. G.Johnson, R. G. Bell, Science, 1998, 280, 705.

Preferably, the transition metal is nickel, iron or cobalt.

Unlike the metal nitrates and similar salts used in the past,transitionmetal compounds used in this invention such as formats or oxalates aredecomposable to the metal by heating without reduction, e.g. under anon-reducing atmosphere.

Suitable carbon-containing compounds for use as the carbon sourceinclude carbon monoxide and hydrocarbons, including aromatichydrocarbons, e.g. benzene, toluene, xylene, cumene, ethylbenzene,naphthalene, non-aromatic hydrocarbons, e.g. methane, ethane, propane,butane, pentane, hexane, cyclohexane, ethylene, propylene or acetylene,and oxygen-containing hydrocarbons, e.g. formaldehyde, acetaldehyde,acetone, methanol or ethanol, or a mixture of two or more thereof. Inpreferred embodiments, the carbon-containing compound is carbon monoxide(CO), methane, ethylene or acetylene.

Preferably, the gaseous carbon source is passed over the substrateparticles.

The carbon source may be mixed with one or more gases acting as adiluent such as inert gases, e.g. argon. The carbon source may also bemixed with non carbon containing gases which play no direct role in thenanotube forming reaction but which play a contributory role, forinstance by reacting with amorphous carbon as it is formed (as aby-product) and so keeping the reaction sites on the catalyst clean andavailable for nanotube formation.

Gases which may be mixed with the carbon source include argon, hydrogen,nitrogen, ammonia, carbon dioxide or helium.

Preferred gas pressures for the gaseous carbon source and optionaldiluent are from 0.1 to 50 bar A, preferably from 0.5 to 5 bar A, morepreferably 1 to 2 bar A. The gaseous effluent from the furnace may berecycled, with or without clean up.

The substrate particles are preferably a suspended finely dividedsubstrate material. In the most straightforward case, the substrateparticles are simply finely ground powders, for example oxides particlesor silicate particles such as silica, alumina, CaSiO_(x), calcium oxideand/or magnesium oxide. Finer materials such as gels and aerogels may begenerated by a range of methods known to those skilled in the art, suchas fuming, colloidal processing, spray-drying, hydrothermal processingand so on. Particular benefit for the production of nanotubes may bederived by using structured substrate particles, particularly mesoporoussilicas, anodised alumina membranes, or zeolites. These materials havechannels of similar dimensions to nanotubes, and can further guide boththe deposition of catalyst and synthesis of nanotubes.

The finely divided substrate particles preferably have a size notsmaller than 1 nm, e.g. not less than 5 nm. They may contain not lessthan 10 atoms, e.g. not less than 15 to 20 atoms, perhaps not less than50 atoms or 75 atoms.

Preferably, the transition metal compound is decomposed by heating, forexample by heating to a temperature between 200° C. and 1000° C., morepreferably between 600° C. and 1000° C. To stimulate decomposition ofthe catalyst precursor material, an additional energy source (over andabove the temperature of the furnace) may be locally applied. The sourcemust be able to penetrate the loaded substrate powder. Such a source ispreferably an intense light source, for example a laser or an intensenon-coherent light source such as a flash discharge lamp. Preferably,the light source is an ultraviolet light source. Alternatively, theadditional energy source may be a plasma discharge or an arc dischargeformed in the presence of the catalyst precursor material.

Preferably, the carbon nanoparticles are carbon nanotubes. Morepreferably, the carbon nanotubes are single walled carbon nanotubes.

Preferably, the method further comprises the initial step ofimpregnating the substrate particles with the transition metal compound.More preferably, where a transition metal salt is used, impregnation isachieved using a solution of the transition metal salt.

Preferably, the method is continuous. In this case, the method maycomprise the steps of:

-   -   continuously providing substrate particles;    -   fluidising the substrate particles with a flow of gaseous carbon        source;    -   heating the transition metal compound on the substrate        particles; and    -   collecting the carbon nanoparticles formed by elution.

Alternatively, the method may comprise the steps of:

-   -   continuously providing substrate particles to an upper part of        an inclined surface;    -   contacting the substrate particles on the inclined surface with        a flow of gaseous carbon source;    -   heating the transition metal compound on the substrate        particles; and        collecting carbon nanoparticles formed from a lower part of the        inclined surface.

In a second aspect, the second invention relates to a method ofproduction of carbon nanoparticles, comprising the steps of:

-   -   providing on substrate particles a transition metal oxalate,        formate or multi metal atom carbonyl;    -   heating the transition metal oxalate, formate or multi metal        atom carbonyl on the substrate particles;    -   contacting a gaseous carbon source with the substrate particles;        and    -   collecting the carbon nanoparticles formed.        The features described in connection with the first aspect of        the invention may also be used in connection with the second        aspect of the invention. In a particularly preferred embodiment        of the second aspect of the invention, the transition metal        oxalate, formate or multi metal atom carbonyl is nickel formate        and the substrate particles comprise silica.

In a third aspect, the present invention relates to carbon nanoparticlesformed by a method as described above.

The invention will be further illustrated by the following non-limitingexamples, with references to the drawings, in which:

FIG. 1 shows the apparatus used in Examples 1, 3 4 and 6;

FIG. 2 shows the product of Example 1:

-   -   a) SEM image    -   b) HRTEM image;

FIG. 3 shows a Raman spectrum of the product of Example 2;

FIG. 4 shows the product of Example 3:

-   -   a) SEM image    -   b) HRTEM image; and

FIG. 5 shows the product of Example 4.

APPARATUS

The apparatus 10 used in Examples 1, 3, 4 and 6 for the growth of carbonnanotubes is shown in FIG. 1. The apparatus 10 comprises sources ofargon 12, acetylene 14, hydrogen 16 and methane 18, connected to flowmeters 20, 22, 24 and 26 respectively. Supply pipes 28, 30, 32 and 34lead from sources 12, 14, 16 and 18 into a drying container 36containing calcium chloride. The outlet 38 of the drying container 36 isconnected to a horizontal furnace 40 with heating elements 42, 44.Catalyst 46 contained in an alumina crucible 48 is positioned in thefurnace 40 aligned with the heating element 44. The outlet 50 of thefurnace passes through a paraffin bubbler 52 and a beaker 54 containingactivated carbon.

In Example 2, a modified version of the apparatus 10 of FIG. 1 is used,wherein the furnace 40 is replaced by a quartz tube (not shown).

EXAMPLE 1

100 mg of silica powder (fumed silica from Aldrich Chemical Company,surface area 200 m²/g) was placed in a flask. To this was added 4.0 mlof aqueous nickel formate (1.09×10⁻² M). The mixture was stronglystirred at room temperature for 30 minutes, then dried in an oven at 90°C. for 16 hours. The sample prepared in this way contained 2.5 wt %nickel loading relative to the silica support. The sample was groundgently by hand in an agate mortar to produce a uniform powder beforeintroducing into the CVD furnace 40 (FIG. 1) in an alumina crucible 48for the growth of CNTs. The growth of CNTs was performed using anargon-methane atmosphere (1:1 argon to methane) with a total gas flowrate of 400 ml/minute. The temperature was 860° C. After 30 minutes theproducts were characterised by scanning electron microscopy (SEM), highresolution transmission electron microscopy (HRTEM), and Ramanspectroscopy. FIG. 2 shows the SEM and HRTEM images of the nanotubes.The nanotubes were clean, pure singlewalled and almost monodisperse,with a diameter of about 1.0 nm. The nanotubes were mainly in the formof small bundles although some individual nanotubes were observed.

EXAMPLE 2

500 mg silica powder (fumed silica from Aldrich Chemical Company,surface area 200 m²/g) was placed in a 100 ml beaker. To this was added25 ml aqueous nickel formate (1.08 ×10⁻² M). The mixture was stronglystirred at room temperature for 30 minutes, followed by further stirringat higher temperature (90° C.) for about 20 hours to dry the sample. Themetal loading in the prepared sample was 3.2 wt % relative to the silicasupport. For nanotube growth, 26.49 mg of the sample was placed in thecentre of a quartz tube preheated to 860° C. under argon atmosphere.Methane was then immediately introduced (1:1 argon to methane volumeratio, total flow rate of 400 ml/min) to initiate the nanotube growth.After 30 minutes of reaction, 25.37 mg of product was obtained. Ramanspectroscopy indicated that the nanotubes were single-walled andmonodisperse (FIG. 3). The yield of the carbon nanotubes was 4.7%, basedon the formula:Yield %=(m ₁ −m ₀)/m ₀×100%

where: m₁ is mass of the catalyst powder after growth

-   -   m₀ is mass of the catalyst powder after heating under identical        conditions as for nanotube growth, except that no methane is        introduced.

EXAMPLE 3

20 mg silica fine powder (fumed silica from Aldrich Chemical Company,surface area 200 m²/g) was placed in a flask. To this was added 1.5 mlaqueous nickel formate (3.8×10⁻³ M) The mixture was strongly stirred atroom temperature for 30 minutes, then dried in an over at 90° C. for 16hours. The prepared sample contained 1.7 wt % nickel loading relative tothe silica support. The sample was ground gently by hand in an agatemortar to produce a uniform powder before introduction into the CVDfurnace 40 (FIG. 1) in an alumina crucible 48 for the growth of CNTs.The growth of CNTs was performed at 650° C. for 60 minutes using anacetylene and argon mixture (1:10 acetylene to argon). The total gasflow rate was 220 ml/minute. The products were characterised by SEM andHRTEM. FIG. 4 shows SEM and HRTEM images of the MWNTs produced. Thediameters of the nanotubes produced were typically about 10 nm.

EXAMPLE 4

When hydrogen (5:5:1 ratio of argon to hydrogen to acetylene, total flowrate of 220 ml/min)was added to the gas stream of Example 3 duringgrowth, high yields of MWNTs were obtained (FIG. 4). The diameters ofthe nanotubes were found to increase to about 22 nm.

EXAMPLE 5

Nickel 38 carbonyl cluster is prepared from smaller clusters by reactionin base or with a molecular template. For example, nickel 38 carbonylcluster is prepared from nickel tetracarbonyl via nickel 6 carbonylcluster as follows. Nickel tetracarbonyl (5 ml) and potassium hydroxide(13 g) in methanol were stirred for 24 h under nitrogen. The resultingdeep red suspension was evaporated under vacuum and the residue wasdissolved in water. Upon addition of solid potassium bromide (20 g) andfurther evaporation under vacuum to eliminate traces of methanol, redmicrocrystals of K₂[Ni₆(CO)₁₂].xH₂O (nickel 6 carbonyl cluster) wereobtained. To synthesise nickel 38 cluster, nickel 6 carbonyl cluster istreated with either platinum (II) chloride or K₂PtCl₄ in 1:1 molar ratioin acetonitrile. The Ni₃₈Pt₆ carbonyl cluster salt can be separated fromimpurities as a result of differential solubility in the solvent.

EXAMPLE 6

100 mg of silica powder (fumed silica from Aldrich Chemical Company,surface area 200 m²/g) is placed in a flask. To this is added 4.0 ml ofdichloromethane dissolved nickel 38 carbonyl cluster (1.09×10⁻² M). Themixture is strongly stirred at room temperature for 30 minutes, thendried in an oven at 90° C. for 16 hours. The sample is ground gently byhand in an agate mortar to produce a uniform powder before introducinginto the CVD furnace 40 (FIG. 1) in an alumina crucible 48 for thegrowth of CNTs. The growth of CNTs is performed using an argon-methaneatmosphere (1:1 argon to methane) with a total gas flow rate of 400ml/minute. The temperature is 860° C. After 30 minutes the products arecharacterised by scanning electron microscopy (SEM), high resolutiontransmission electron microscopy (HRTEM), and Raman spectroscopy.

The one step process of the Examples is simple and cheap. Thecombinations of precursor and support particles used are particularlyeffective at producing high yields of pure nanotubes.

Whilst the applicant does not wish to be bound by this theory, it isbelieved that the precursor molecules are adsorbed onto the surface ofthe support particles. At high temperatures the transition metal saltprecursor of Examples 1 to 4 thermally decomposes to produce metalnanoparticles of 1-3 nm diameter and gaseous side products. Suchnanoparticles are chemically very active and serve as a catalyst for CNTgrowth. Transition metal carbonyl cluster precursors decompose by lossof the surface and connecting carbonyl ligands such that each clusterforms a metal nanoparticle. Each metal nanoparticle is of suitable sizeto nucleate an individual nanotube.

In general larger catalyst particles suppress single wall carbonnanotubes. Large catalyst particles are produced by sintering. Thisoccurs if the catalyst is held at high temperature, particularly wherethere are high metal loadings or a smooth surface on which metalparticles are mobile. Where sintering occurs, multi wall carbonnanotubes or encapsulated nanoparticles tend to be formed, probablydepending on the size difference between the catalyst and thefundamental support particles. A strong attraction between substrate andcatalyst favours nanotube growth. Temperatures which are too high leadto overcoating and production of multi wall carbon nanotubes, nanofibresor vapour grown carbon fibres. In the processes of the examples there isno need for additional heat treatment that can give rise to undesirablesintering effects.

The processes of the invention as illustrated above have severaladvantages over known CVD processes. First, in the processes of theExamples, there is no need for the usual calcination and reduction stepsto produce the catalyst from the catalyst precursor. Secondly, water isused as the solvent in preparation of the catalyst precursors ofExamples 1 to 4, and may also be used in the preparation of catalystprecursors from transition metal carbonyl salts. The use of water ratherthan organic solvents has cost and environmental benefits. Thirdly, theprecursors of Examples 1 to 4 are particularly easy to handle and tointroduce onto the substrate as a result of their solubility in water,stability, low volatility, low cost and low toxicity. By comparison,conventional precursors such as nickel tetracarbonyl are difficult tohandle.

The process of Example 6 has the advantage that each transition metalcarbonyl cluster forms a metal nanoparticle of defined size. This givesoptimal control over nanotube diameter and may allow chirality control.

The process of the Examples can be used to produce high-quality, highyield SWCNTs on an industrial scale. The process may be scaled up in anumber of ways. For example, a bed of supported catalyst may befluidised using suitable carbon-containing gas and reacted productcollected by elution due to its reduced effective density, whilst freshsupported catalyst precursor is continuously added. Alternatively, thesupported catalyst precursor may be introduced to the top of a rotating,gently sloping cylindrical furnace tube and allowed to travel down thelength of the heated furnace under a suitable atmosphere, growingnanotubes during the well-defined transit time. The quality of thenanotubes, in terms of diameter, length, ratio of SWNTs to MWNTs andamorphous carbon, can be controlled by controlling the catalyst loading,dispersion level in supports and the CVD growth parameters.

Although the invention has been described with reference to theExamples, it will be appreciated that various modifications are possiblewithin the scope of the invention.

1. A continuous method of production of carbon nanotubes, comprising:continuously providing substrate particles; providing on the substrateparticles a transition metal compound that is a nickel, iron or cobaltformate which is decomposable to yield a transition metal catalyst undera non-reducing atmosphere permitting carbon nanotube formation;fluidizing the substrate particles with a flow of gaseous carbon source;heating the transition metal formate on the substrate particles; before,during or after contacting the gaseous carbon source with the substrateparticles, decomposing the transition metal formate to yield thetransition metal catalyst on the substrate particles by heating withoutthe transition metal catalyst being reduced by the atmosphere; formingsingle walled carbon nanotubes by decomposition of the carbon sourcecatalysed by the transition metal catalyst; and collecting the singlewalled carbon nanoparticles formed by elution.
 2. A method as claimed inclaim 1, wherein the gaseous carbon source is a hydrocarbon or carbonmonoxide.
 3. A method as claimed in claim 2, wherein the gaseous carbonsource is methane or acetylene.
 4. A method as claimed in claim 1,wherein the gaseous carbon source is passed over the substrateparticles.
 5. A method as claimed in claim 1, wherein the gaseous carbonsource is mixed with a diluent.
 6. A method as claimed in claim 5,wherein the diluent is argon.
 7. A method as claimed in claim 1, whereinthe substrate particles comprise oxide particles and/or silicateparticles.
 8. A method as claimed in claim 7, wherein the substrateparticles comprise one or more of silica, alumina, CaSiO_(x), calciumoxide or magnesium oxide.
 9. A method as claimed in claim 1, wherein thesubstrate particles are in the form of a fumed powder, a colloid, a gelor an aerogel.
 10. A method as claimed in claim 1, wherein thetransition metal compound is decomposed by heating to a temperaturebetween 200° C. and 1000° C.
 11. A method as claimed in claim 10,wherein the transition metal compound is decomposed by heating to atemperature between 600° C. and 1000° C.
 12. A method as claimed inclaim 1, further comprising an initial step of impregnating thesubstrate particles with the transition metal compound.
 13. A method asclaimed in claim 1, comprising: continuously providing substrateparticles to an upper part of an inclined surface; contacting thesubstrate particles on the inclined surface with a flow of gaseouscarbon source; heating the transition metal compound on the substrateparticles; and collecting single walled carbon nanotubes formed from alower part of the inclined surface.
 14. A method as claimed in claim 1,wherein heating the transition metal compound on the substrate particlesto decompose the transition metal compound to the transition metalcatalyst occurs before contacting the substrate particles with the flowof gaseous carbon source.
 15. A method as claimed in claim 14, whereinthe transition metal compound is nickel formate and the substrateparticles are silica particles.